Introduction

The Self-contained System (SCS) approach is an architecture that
focuses on a separation of the functionality into many independent
systems, making the complete logical system a collaboration of many
smaller software systems. This avoids the problem of large monoliths
that grow constantly and eventually become unmaintainable.
Over the past few years, we have seen its benefits in many mid-sized
and large-scale projects.

The idea is to break a large system apart into several smaller
self-contained systems, or SCSs, that follow certain rules.

SCS Characteristics

Each SCS is an autonomous web
application. For the SCS's domain, all data, the logic to process that data and
all code to render the web interface is contained within the
SCS. An SCS can fulfill its primary use cases on its own,
without having to rely on other systems being available.

Each SCS is owned by one team. This
does not necessarily mean that only one team can change the
code, but the owning team has the final say on what goes
into the code base, for example by merging pull-requests.

Communication with other SCSs or 3rd party
systems is asynchronous wherever possible. Specifically,
other SCSs or external systems should not be accessed synchronously
within the SCS's own request/response cycle. This decouples the
systems, reduces the effects of failure, and thus supports
autonomy. The goal is decoupling concerning time: An SCS should
work even if other SCSs are temporarily offline. This can be
achieved even if the communication on the technical level is
synchronous, e.g. by replicating data or buffering requests.

An SCS can have an optional service
API. Because the SCS has its own web UI, it can
interact with the user — without going through a UI
service. However, an API for mobile clients or for other SCSs
might still be useful.

Each SCS must include data and
logic. To really implement any meaningful features both
are needed. An SCS should implement features by itself and
must therefore include both.

An SCS should make its features usable
to end-users via its own UI. Therefore the SCS should have
no shared UI with other SCSs. SCSs might still have
links to each other. However, asynchronous
integration means that the SCS should still work even if
the UI of another SCS is not available.

To avoid tight coupling an SCS should
share no business code with other SCSs. It might be fine to
create a pull-request for an SCS or use common libraries,
e.g. database drivers or oAuth clients.